The ability to conduct high-speed data communications between relatively remote data processing systems and associated subsystems is currently a principal requirement of a variety of industries and applications, such as business, educational, medical, financial and personal computer users. Moreover, it can be expected that present and future applications of such communications will continue to engender more such systems and services. One technology that has attracted particular interest in the telecommunication community is digital subscriber line (DSL) service. DSL technology enables a public switched telephone network (PSTN) to use existing telephone copper wiring infrastructure to deliver a relatively high data bandwidth digital communication service, that is selected in accordance with expected data transmission rate, the type and length of data transport medium, and schemes for encoding and decoding data.
A reduced complexity diagram of the architecture of such a DSL system is shown in FIG. 1 as having mutually compatible digital communication transceivers 1 and 3 respectively installed at remotely separated ‘west’ and ‘east’ sites 2 and 4, and coupled to a communication link 10, such as a twisted pair of an existing copper plant. One of these transceivers, the west site transceiver 1, for example, may reside within a digital subscriber line access multiplexer (DSLAM) 6 of a network controller site (such as a telephone company central office (CO)).
The DSLAM 6 is coupled with an associated network backbone 5 providing access to various information sources 7 and the Internet 8. As such, west site transceiver 1 is used for transport of digital communication signals, such as asynchronous transfer mode (ATM)-based packetized voice and data, from the west central office site 2 over the communication link 10 to an integrated access device (IAD), which serves as the DSL transceiver 3 at the east end of the link, and is typically coupled with a computer 9 at a customer premises, such as a home or office. The IAD consolidates digitized data, voice and video traffic over a common wide area network (WAN) DSL link. The digitized voice stream may be digitally encoded as mu-law or a-law voice samples, as may be supplied by an industry standard ITU G.711 codec, or it may comprise digitally encoded voice samples from an integrated services digital network (ISDN) phone.
When digitally encoded voice samples are encapsulated in accordance with packet or cell protocol for network transport (for example, using voice over asynchronous transfer mode (ATM) or voice over internet protocol (IP)), it is often desirable that the IAD incorporate both echo cancellation and compression processing, for the purpose of both optimizing signal quality and maximizing the bandwidth available for non-voice signaling. Commonly employed industry standard signal processing operators include ITU G.168 echo cancellation and ITU G.726 adaptive differential pulse code modulation (ADPCM) compression.
Pursuant to the invention described in the above-referenced '375 application, these signal processing operators are implemented in a cascaded architecture diagrammatically illustrated in FIG. 2. As shown therein, an array of codecs 30 that are coupled to associated POTS phones 32 output respective digitally encoded voice signals as part of a time division multiplexed stream over a serial communication link 34.
Rather than terminate the serial communication link 34 at a DSP array as has been conventional practice, the architecture of FIG. 2 cascades respective echo cancellation and compression engines 40 and 50 within the TDM transport path 34, and then outputs the processed voice sample data produced by the cascaded signal processing operators in TDM format for application to a communication co-processor 35 within (or attached to) a downstream host processor 36. The processor assembles the incoming voice sample data into packets in accordance with encapsulating protocol, and outputs the packetized voice signal stream over a digital communication link to a destination receiver device.
For ATM-based voice over data transmissions, as a non-limiting example, the processed voice sample data may be encapsulated using respective algorithms to produce an AAL2 header and an ATM header. A control bus 39 is coupled between host processor 35 and the signal processing engines for supervisory control communications, and establishing operational parameters, as in a conventional communication signal processing application. However, it is not used for data transport.
The processed voice sample data received by the communication co-processor 35 from the TDM link 34 may be encapsulated using a direct memory access (DMA)-based packet generation mechanism of the type described in U.S. patent application Ser. No. 10/095,380, filed Mar. 12, 2002, by P. Herron et al, entitled: “Mechanism for Utilizing Voice Path DMA in Packetized Voice Communication System to Decrease Latency and Processor Overhead,” assigned to the assignee of the present application and the disclosure of which is incorporated herein.
The respective echo cancellation and compression engines 40 and 50 are interfaced with the TDM bus 34 on their input (upstream) and output (downstream) ends by way of two full TDM ports. These ports may be configured as serial-to-parallel and parallel-to-serial conversion and associated signal encoding format (e.g., mu-law) circuits of the type conventionally employed in the art for the purpose. This allows each of the echo cancellation and compression operators to operate directly on the data transported by any channel of the TDM voice sample signal stream, and produces processed digitized voice signal data that is then placed back in the same channel of the TDM stream for transport directly to the communication co-processor 35. This avoids burdening the host processor 36 with the substantial data interfacing exercise of having to use data bus cycles to extract the data, as in a conventional DSP array-based architecture.
Now although the cascaded echo canceler and compression operator arrangement described in the '375 application performs its function as intended on the incoming TDM signal streams, it has been found that operational anomalies may occur, depending upon the equipment with which the system is interfaced. As a non-limiting example, it has been found that, in some instances, as a result of an inadvertent false operation by the echo canceler, a user who has gone off-hook and received dial tone from the network switch may be unable to break the dial tone signal by sending a DTMF tone back toward the switch. It is desirable, when an anomaly such as but not limited to this situation arises, that it be possible to debug the problem without having to send a technician to the customer site.